A spark plug is used for an ignition of an internal combustion engine such as an automobile engine. In general, the spark plug includes; a tubular metal shell; a tubular insulator which is disposed in an inner hole of the metal shell; a center electrode which is disposed in an inner hole of the leading end of the insulator; and a ground electrode in which one end is bonded to the leading end of the metal shell and the other end thereof forms a spark discharge gap between the ground electrode and the center electrode. In addition, the spark plug is spark-discharged at the spark discharge gap formed between the leading end of the center electrode and the leading end of the ground electrode in a combustion chamber of an internal combustion engine, and burns fuel filled in the combustion chamber.
However, in recent years, according to an output improvement by a supercharger, technology which lengthens the distance that can be traveled using a small amount of fuel has been developed. In this kind of internal combustion engine, temperature within the combustion chamber tends to be increased, and particularly, the temperature in the vicinity of an area, in which the leading end of the ground electrode is positioned, tends to be a high temperature. Moreover, according to miniaturization of the spark plug, the ground electrode is also thin. Therefore, the ground electrode cannot conduct heat generated by the discharge of the spark plug to escape to the metal shell (also referred to as “heat conduction”). As a result, the temperature of the ground electrode itself is also easily increased.
The spark plug is used under a high temperature environment as described above, if the spark plug has a configuration in which the temperature of the ground electrode is also easily increased, it is difficult to maintain a desired performance using the spark plug of the related art.
In Patent Document 1 having an object of providing a spark plug capable of decreasing a temperature increase of a ground electrode and of suppressing an extinction action thereof, a spark plug is disclosed in which a core having higher thermal conductivity than that of the ground electrode is embedded in at least a portion other than a curved portion of the ground electrode.
In Patent Document 2 having an object of providing an electrode material for a spark plug having excellent characteristics in oxidation resistance, spark wear resistance, and manufacturability, the following is disclosed. That is, it is necessary to enhance the thermal conductivity to improve the oxidation resistance of an alloy for the spark plug, and it is effective to enhance a melting pointing to improve the spark wear resistance. Thus, in order to simultaneously satisfy two necessary characteristics, in an electric material formed by high Ni-based alloy, adding a small amount of Si, adding a small amount of Hf and/or Re, decreasing Mn and Al, and adding one or more kinds of rare earth elements and/or Y by a small amount are simultaneously performed.
However, since there is a tendency for the inner portion of the combustion chamber to reach increasingly higher temperatures and for the spark plug to be miniaturized, a ground electrode having improved heat conduction is needed.